The application of synthetic polymer chemistry to the field of sports equipment has revolutionized the performance of athletes in many sports. One sport in which this is particularly true is golf, especially as relates to advances in golf ball performance and ease of manufacture. For instance, the earliest golf balls consisted of a leather cover filled with wet feathers. These “feathery” golf balls were subsequently replaced with a single piece golf ball made from “gutta percha,” a naturally occurring rubber-like material. In the early 1900's, the wound rubber ball was introduced, consisting of a solid rubber core around which rubber thread was tightly wound with a gutta percha cover.
More modern golf balls can be classified as one-piece, two-piece, three-piece or multi-layered golf balls. One-piece balls are molded from a homogeneous mass of material with a dimple pattern molded thereon. One-piece balls are inexpensive and very durable, but do not provide great distance because of relatively high spin and low velocity. Two-piece balls are made by molding a cover around a solid rubber core. These are the most popular types of balls in use today. In attempts to further modify the ball performance especially in terms of the distance such balls travel and the feel transmitted to the golfer through the club on striking the ball, the basic two piece ball construction has been further modified by the introduction of additional layers between the core and outer cover layer. If one additional layer is introduced between the core and outer cover layer a so called “three-piece ball” results and similarly, if two additional layers are introduced between the core and outer cover layer, a so called “four-piece ball” results, and so on.
Wound balls typically have either a solid rubber, or liquid-filled, center around which many yards of a stretched elastic thread or yarn is wound to form a core. The wound core then is covered with a durable cover material, e.g., an ionomer or other thermoplastic material or a softer cover such as balata or cast polyurethane. Wound balls generally are softer than two-piece balls, and they provide more spin, which enables a skilled golfer to have more control over the ball's flight. In particular, it is desirable for the golfer to be able to impart backspin to the ball, for purposes of controlling its flight and controlling the action of the ball upon landing on the ground. For example, substantial backspin will make the ball stop once it strikes the landing surface instead of bounding forward. The ability to impart backspin onto a golf ball is related to the extent to which the golf ball's cover deforms when it is struck by a golf club. Because conventional wound balls are generally more deformable than are conventional two-piece balls, it is easier to impart spin to wound balls. However, higher spinning wound balls typically travel a shorter distance when struck, as compared to two-piece balls. Moreover, because wound balls generally have a more complex structure, they generally require a longer time to manufacture and are more expensive to produce than are two-piece balls. Golf balls having a two-piece construction generally are most popular with the recreational golfer, because they are relatively durable and provide maximum distance. Two-piece balls have a single solid core, usually formed of a cross-linked rubber, which is encased by a cover. Typically, the solid core is made of polybutadiene, which is chemically cross-linked with peroxide, or sulfur compounds together with co-cross-linking agent, such as zinc diacrylate. The cover of such balls often comprises tough, cut-proof blends of one or more materials known as ionomers, which typically are ethylene/acrylic acid copolymers or ethylene/acrylic acid/acrylate terpolymers in which some or all of the acid groups are neutralized with metal cations. Such ionomers are commercially available under trademarks such as SURLYN®, which are resins sold commercially by DuPont, of Wilmington, Del., or IOTEK®, which is sold commercially by ExxonMobil, of Irving, Tex.